If North Korea launched a nuclear attack, the death toll would be costly: perhaps as bad as 2.1 million deaths in Tokyo and Seoul alone.

In the event of an “unthinkable” escalation, casualties in the East Asian capitals of key American allies would be catastrophic, including as many as 7.7 million injuries, according to a new report from 38 North, a North Korea analysis group based at Johns Hopkins University’s US-Korea Institute.

Since 2011, North Korea has carried out 98 ballistic missile tests and six underground nuclear tests overall. The most recent, on September 3, clocked in around 120 kilotons and North Korea was quick to claim it had successfully tested a hydrogen bomb. The tests have also revealed the isolated state’s increasing technical sophistication: on July 4 and July 28, North Korean state media said it had tested intercontinental ballistic missiles (ICBMs) capable of reaching the mainland US

The report offers hypothetical scenarios based on the assumption that North Korea has a nuclear arsenal of some 20-25 warheads. The warheads are estimated to range from 15 kilotons about the size of the “Little Boy” bomb dropped on Hiroshima in 1945, killing more than 200,000 to 250 kilotons the estimated strength of a thermonuclear weapon. The report suggests that were North Korea to launch its entire arsenal against Tokyo (population 37.9 million) and Seoul (24.1 million), casualties in each city could reach as high as 3.8 million.

The report cautions that most nuclear weapons systems don’t have 100 percent reliability, and America’s allies have defenses South Korea has deployed the US Terminal High Altitude Air Defense (THAAD) system despite the initial opposition of new South Korean President Moon Jae-in, while Japan plans to install an Aegis Ashore Ballistic Missile defense system. But Tokyo and Seoul are far more densely populated than they were during World War II or the Korean War (1950-1953), and the latter is in reach of North Korea’s conventional weapons, including artillery, meaning a devastating death toll in any all-out conflict would be certain, according to 38 North.